Water has always been present on Earth, in one form or another. It was present, as ice and in water-bearing clay minerals in the planetesimals that the Earth was built from. Some water, in the form of vapor and ice crystals, has always been present in the atmosphere of the Earth. Water vapor is also released from the Earth's interior by volcanoes.

But, I am being a little coy . . . I am guessing that you are wondering when Earth first had liquid water. The answer is probably earlier than you think! The Earth is about 4.568 billion years old. It's difficult to study what the Earth was like back then because the Earth's crust is constantly being recycled, leaving very few rocks older than ~3.5 billion years. However, we have found zircon minerals that are 4.37 billion years old, eroded from ancient rock formations of the early Earth, and found at the bottom of a river bed in Australia. These minerals show evidence that the surface of the Earth was solid and cool when this mineral solidified, just 165 million years after the Earth was formed!

There was even liquid water present where this rock solidified!

So as far back as we have geological evidence of what the Earth was like, there was liquid water present.

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